Idiosyncratic drug reactions, also known as
type B reactions, are
drug reactionsAn adverse drug reaction is an expression that describes harm associated with the use of given medications at a normal dosage. ADRs may occur following a single dose or prolonged administration of a drug or result from the combination of two or more drugs...
that occur rarely and unpredictably amongst the population. This is not to be mistaken with
idiopathicIdiopathic is an adjective used primarily in medicine meaning arising spontaneously or from an obscure or unknown cause. From Greek ἴδιος, idios + πάθος, pathos , it means approximately "a disease of its own kind". It is technically a term from nosology, the classification of disease...
, which implies that the cause is not known. They frequently occur with exposure to new drugs, as they have not been fully tested and the full range of possible side-effects have not been discovered; they may also be listed as an
adverse drug reactionAn adverse drug reaction is an expression that describes harm associated with the use of given medications at a normal dosage. ADRs may occur following a single dose or prolonged administration of a drug or result from the combination of two or more drugs...
with a drug, but be extremely rare.
Some patients have multiple-drug intolerance. Patients who have multiple idiopathic effects that are nonspecific are more likely to have anxiety and depression.
Idiosyncratic drug reactions appear to not be concentration dependent. A minimal amount of drug will cause an immune response, but it is suspected that at a low enough concentration, a drug will be less likely to initiate an immune response.
Mechanism
In adverse drug reactions involving overdoses, the toxic effect is simply an extension of the pharmacological effect (Type A adverse drug reactions). On the other hand, clinical symptoms of idiosyncratic drug reactions (Type B adverse drug reactions) are different than the pharmacological effect of the drug.
The proposed mechanism of most idiosyncratic drug reactions is immune mediated toxicity. To create an immune response, you must have a foreign molecule that antibodies can bind to (ie. the antigen) and you must have cellular damage. Very often, drugs will not be immunogenic because they are too small to bind antibodies. However, a drug can cause an immune response if the drug binds a larger molecule. Some unaltered drugs such as
penicillinPenicillin is a group of antibiotics derived from Penicillium fungi. They include penicillin G, procaine penicillin, benzathine penicillin, and penicillin V....
will bind avidly to proteins. Others must be bioactivated into a toxic compound that will in turn bind to proteins. The second criteria of cellular damage can come either from a toxic drug/drug metabolite or from an injury or infection.
These will sensitize the immune system to the drug and cause a response.
Idiosyncratic reactions fall conventionally under toxicology.